Electrochemistry Communications 6 (2004) 115–119

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Epitaxial growth and magnetic properties ofelectrochemically multilayered [CoPtP/Cu]n films

Kwan H. Lee *, Gyeung H. Kim, Won Y. Jeung

Materials Research Division, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-791, Republic of Korea

Received 1 October 2003; received in revised form 20 October 2003; accepted 21 October 2003

Published online: 14 November 2003

Abstract

Electrochemically fabricated [CoPtPð100 nmÞ/Cuðx nmÞ] multilayers and their resulting magnetic properties were investigated. It was

observed that the [CoPtPð100 nmÞ/Cuðx nmÞ] multilayer had an epitaxial microstructure in which each CoPtP and Cu layer existed

coherently as if they were one single layer. Moreover, their magnetic properties varied considerably according to the thickness of the

Cu layers. Microstructural characterization of the [CoPtPð100 nmÞ/Cuðx nmÞ] multilayers by transmission electron microscopy revealed

that the thicker the Cu layers, the more significantly aligned the [1 1 1]fcc direction of the Cu layers, a situation which strongly

induced the growth of the CoPtP layers in the direction of [0 0 2]hex. These microstructural features provide a reasonable explanation

for the correlation of the magnetic properties of the electrodeposited [CoPtPð100 nmÞ/Cuðx nmÞ] multilayers with the thickness of the

Cu layers, because magneto-crystalline anisotropy could account for the enhancement of the perpendicular magnetic properties of

the films.

� 2003 Elsevier B.V. All rights reserved.

Keywords: CoPtP; Multilayer; Electrodeposition; Epitaxial growth; Magnetic properties

1. Introduction

Recently, the electrodeposition process has been in-creasingly used in those fields requiring high perfor-

mance magnetic materials, such as MRAM (magnetic

random access memory), ultrahigh density perpendicu-

lar recording media, and NEMS/MEMS (nano/micro

electro mechanical systems), which are used for fabri-

cating devices with the dimensions in nanometers/mi-

crons [1–5]. In these areas, the electrodeposition process

has regained its popularity, mainly due to its inherentability to fill up high aspect ratio patterns and to tailor

the magnetic properties to specific needs, as compared

with conventional vacuum evaporation techniques such

as CVD and PVD [3–5].

Thin film CoPtP alloys are known as one of the fer-

romagnetic alloys with the best PMA (perpendicular

magnetic anisotropy) among those magnetic alloys

which can be prepared from electrodeposition. There-

* Corresponding author. Tel.: +82-2-958-6804; fax: +82-2-958-6839.

E-mail address: kwanhyi@kist.re.kr (K.H. Lee).

1388-2481/$ - see front matter � 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2003.10.022

fore, extensive studies have been performed on the

electrochemical fabrication of CoPtP alloys, in order to

take advantage of their superior magnetic propertiesand their relatively simple fabrication process [6–8].

However, it is also known that when the thickness of the

CoPtP film exceeds about 1 lm, its inherently high PMA

rapidly deteriorates with increasing thickness, due to the

formation of a columnar structure with larger grains [7].

Based on this result, it might be expected that the su-

perior PMA of the CoPtP film would be maintained or

even improved, if it were applied in the form of½CoPtPð<1 lmÞ=Cu� multilayers, in which the thickness of

each CoPtP layer was kept to within 1 lm by using Cu

(a typical non-magnetic material) interlayers, and this

hypothesis provided the motivation for this study.

Moreover, we recently observed that the magnetic

properties of electrodeposited [CoPtP/Cu]n films were

also severely conditional on the thickness of the Cu in-

terlayers. Therefore, in order to produce thick magneticfilms having high magnetization and coercivity, we

attempted the electrodeposition of [CoPtP/Cu]n multi-

layer films as an effective method of circumventing the

116 K.H. Lee et al. / Electrochemistry Communications 6 (2004) 115–119

thickness limitation by means of microstructural modi-fication. The microstructural features of the multilayer

structures, including their peculiar epitaxial growth, are

also examined, in order to elucidate the role of the Cu

interlayer thickness and the relationship between the

microstructure and the magnetic properties of the elec-

trodeposited CoPtP multilayers, which will lead to a

plausible explanation for the role of the Cu layers in

controlling the magnetic properties of the electrochem-ically multilayered [CoPtP/Cu] films.

2. Experimental

[CoPtP/Cu]n multilayers were electrochemically pre-

pared by means of the DBT (dual bath technique). Each

layer was galvanostatically electrodeposited at 0.5 A/dm2 and 40 �C. The working electrode (1 cm� 1 cm)

consisted of a 200 nm thick Au layer on a (1 0 0) Si

wafer. Electrolytic cobalt plates (99.9% purity) and a

SCE (Saturated Calomel Electrode) were used as the

counter electrode and the reference electrode, respec-

tively. The bath for the CoPtP layer was composed of

0.12 M CoSO4, 0.45 M Na4P2O7, 0.01 M H2PtCl6 and

0.05 M NaH2PO2, and that for the Cu interlayer wasmade up of 0.3 M CuSO4 and 0.45 M Na4P2O7. All

solutions were prepared using ultra pure deionized water

(over 18 MX at 40� 1 �C). The thickness of each layer

was adjusted by controlling the electrodeposition time,

taking into consideration the current efficiency, which

was ascertained in our preliminary experiments through

SEM (Scanning Electron Microscopy) observation and

weight gain measurements.The magnetic properties of the prepared specimens

were measured by means of a VSM (Vibrating Sample

Magnetometer, 7400, LakeShore, USA). The micro-

structural and crystallographic features of the [CoPtP/

Cu] multilayer films were investigated by means of a

transmission electron microscope (TEM, CM30, Philips,

The Netherlands) operated at 200 kV, through imaging

and electron diffraction, respectively.

Fig. 1. Cross-sectional TEM images of [CoPtP/Cu] multilayers with

different Cu thicknesses; (a) 20 nm, (b) 50 nm, and (c) 100 nm, re-

spectively.

3. Results and discussion

There are two electrochemical methods which can be

used to fabricate multilayered structures: SBT (single

bath technique) and DBT (dual bath technique). SBT

allows the creation of a multilayer through the appli-cation of a pulsed current in a single bath containing

both of the precursors for each layer, while DBT uses

two separate baths each containing one of the two in-

dividual precursors for each layer. DBT was employed

in this study, mainly due to its ability to provide a sharp

interface as well as more homogeneous layers. Cross-

sectional TEM images of the [CoPtPð100 nmÞ/Cuðx nmÞ]

films prepared by the DBT method are shown in Fig. 1.The films consist of 100 nm thick CoPtP layers grown

alternatively with either 20, 50 or 100 nm thick Cu

layers. It was observed that while other types of multi-

layers [9,10] had diffuse or wavy interfaces, the multi-

layers shown in Fig. 1 have well-defined interfaces with a

uniform layer thickness successfully controlled by our

self-made DBT apparatus.

K.H. Lee et al. / Electrochemistry Communications 6 (2004) 115–119 117

Fig. 2 shows the variation of the magnetic hysteresisloops of the [CoPtPð100 nmÞ/Cuðx nmÞ] films with respect

to the thickness of the Cu interlayer. It is apparent in

Fig. 2 that the magnetic properties of the [CoPtPð100 nmÞ/Cuðx nmÞ] films exhibit a strong dependency on the Cu

layer thickness, i.e., the PMA characteristics of the film

increase in proportion to the thickness of the Cu layer.

More specifically, the perpendicular coercivities of the

multilayer films were enhanced from 2770 Oe at tCu ¼20 nm to 4150 Oe at tCu ¼ 100 nm, while the in-plane

coercivities exhibited the opposite variation from 1930

Oe at tCu ¼ 20 nm to 1010 Oe at tCu ¼ 100 nm. The

squareness (M r/M s) showed the same trend with respect

to the thickness of the Cu interlayers as did the PMA.

The changes in magnetic properties of a given mate-

rial are often closely related to its microstructural vari-

ations. Our previous study [1] demonstrated that themagnetic properties of an electrodeposited Co(P) alloy

could be altered by varying the concentration of am-

monium chloride, whose presence causes microstruc-

tural and crystallographic differences and eventually

magnetic property variations even with the same

amount of P in the Co(P) alloys. It is also speculated

that the variation in the thickness of the Cu layers may

bring about unique microstructural variations in the

(b)

tCu

= 20 nm t

Cu = 50 nm

tCu

= 100 nm

Thickness of Cu interlayer

(a)

Mag

netiz

atio

n

Easy Magnetization High Squareness

High Coercivity

c-axisc-axisApplied

Field

(b)(a)

Fig. 2. The variation of the magnetic hysteresis loops measured in the

direction perpendicular (a) and parallel (b) to the film plane according to

the thickness of the Cu layers; 20 nm (dotted line), 50 nm (dashed line),

and 100 nm (solid line), respectively.

multilayered specimens. Microstructural investigationswere carried out by TEM to confirm this hypothesis.

The DF (dark field) images obtained using cross-sec-

tional TEM, which are shown in Fig. 3, reveal the ex-

istence of microstructural differences between the two

[CoPtPð100 nmÞ/Cuðx nmÞ] samples having the Cu layer

thicknesses of 50 and 100 nm. One striking feature of

these multilayered samples is that strong crystallo-

graphic alignment exists throughout the thickness of thefilm, even though each layer is grown successively and

has a different crystal structures. Evidence for this

strong crystallographic alignment is found in the central

region of Fig. 3(b), where a bright band with a width of

about 200 nm extends from the substrate to the top of

the film. This implies that there is an epitaxial or highly

preferred orientation relationship between the Cu and

CoPtP layers. It is also worth mentioning that the widthof the ‘‘aligned region’’ increases with increasing Cu

interlayer thickness, as can be seen by comparing

Fig. 3(a) and (b).

More detailed crystallographic characterization was

performed using electron diffraction analysis of the

multilayered samples. The electron diffraction patterns

obtained from the [CoPtPð100 nmÞ/Cuðx nmÞ] (x¼ 20, 50,

Fig. 3. Dark field images of [CoPtP/Cu] multilayers with Cu layer

thicknesses of (a) 50 nm and (b) 100 nm.

Table 1

Indexing of the electron diffraction pattern shown in Fig. 4(d)

Ring # Inter-planar

spacing (nm)

Index

Cobalt (hex) Copper (fcc)

0.2055 0 0 2 1 1 1

0.1931 1 0 1

0.1775 2 0 0

0.1255 1 1 0

0.1246 2 2 0

0.1152 1 0 3

0.1065 3 1 1

118 K.H. Lee et al. / Electrochemistry Communications 6 (2004) 115–119

100) films are shown in Fig. 4. They provide clear evi-dence for the effect of the Cu layer thickness on the mi-

crostructural modification of the multiplayer films. For

example, the [CoPtPð100 nmÞ/Cuð20 nmÞ] films (Fig. 4(a))

exhibited a completely random crystallographic orien-

tation of both the Cu and CoPtP layers. On the other

hand, in the case of the multilayered films having a Cu

layer thickness of more than 50 nm (Fig. 4(c) and (d)),

there existed a specific orientation relationship betweenthe Cu and CoPtP layers, as well as a strong texture, as

shown by the streaked diffraction spots. An enlarged

diffraction pattern from the [CoPtPð100 nmÞ/Cuð100 nmÞ]film (Fig. 4(d)) was indexed, in order to extract quanti-

tative information on the orientation relationship. Those

diffraction spots, which are numbered and marked with

an arrow, were indexed as listed in Table 1, and it was

interesting to note that the spot marked could be in-dexed as both (0 0 2)hex and (1 1 1)fcc, which suggested

that the (0 0 2) planes of the CoPtP layer and the (1 1 1)

planes of the Cu layers could grow epitaxially. The

analysis of the electron diffraction patterns thus implied

that the predominant growth of the (1 1 1) Cu grains

exerted a direct influence on the preferential growth of

the CoPtP layers, with their (0 0 2) planes parallel to the

direction of growth. Since the easy magnetization di-rection is along the c-axis of the hexagonal structure, thealignment of the c-axis with the direction of an applied

magnetic field gives rise to easier magnetization, i.e., in-

duces high squareness and coercivity. Since the magnetic

Fig. 4. Cross-sectional electron diffraction patterns of [CoPtP/Cu] multilayers

enlarged pattern of (c).

properties shown in Fig. 2(a) were measured in the per-pendicular direction of the film, the c-axis of the hexag-onal CoPtP layer had to be aligned with the direction of

the applied magnetic field for the film with increasing

thickness of Cu interlayers, as shown in Fig. 4. The fact

that the degree of PO (preferred orientation) was stron-

ger in the [CoPtPð100 nmÞ/Cuð100 nmÞ] films than in the

[CoPtPð100 nmÞ/Cuð50 nmÞ] films, as shown in Fig. 4, also

provided a plausible explanation for the experimentalobservation that the multilayer films with tCu ¼ 100 nm

exhibited a superior PMA than those with tCu ¼ 20 and

50 nm in Fig. 2. Therefore, the microstructural features

shown in Fig. 4 provide an accurate explanation for the

magnetic properties observed in Fig. 2.

In summary, varying the thickness and crystallinity of

the Cu layers has an effect on the electrodeposited CoPtP

with different Cu thicknesses: (a) 20 nm, (b) 50 nm, (c) 100 nm and (d)

K.H. Lee et al. / Electrochemistry Communications 6 (2004) 115–119 119

layers, causing microstructural differences to appear inthe form of different grain sizes and textures, which in

turn, manifest themselves in the variation of various

magnetic properties, such as the coercivity and square-

ness. In particular, the epitaxial growth of the Cu and

CoPtP layers induces the strong PO of the [0 0 2]hex of

CoPtP layers, which is a main cause of the superior PMA

characteristics of the films. To the best of our knowledge,

such epitaxial growth of the multilayer electrochemicallyfabricated by DBT has never before been reported.

Acknowledgements

The financial support from the ‘‘R&D Program for

NT-IT Fusion Strategy of Advanced Technologies’’ is

gratefully acknowledged.

References

[1] K.H. Lee, G.H. Kim, W.Y. Jeung, Electrochem. Commun. 4

(2002) 605.

[2] K.H. Lee, H.Y. Lee, W.Y. Jeung, W.Y. Lee, J. Appl. Phys. 91

(2002) 8513.

[3] W.Y. Jeung, D.H. Choi, K.H. Lee, Mater. Sci. 21 (2003) 147.

[4] T.M. Liaksopoulos, W. Zhang, C.H. Ahn, IEEE Trans. Magn. 32

(1996) 5154.

[5] H.J. Cho, C.H. Ahn, IEEE Trans. Magn. 36 (2000) 686.

[6] L. Callegaro, E. Puppin, P.L. Cavallotti, G. Zangari, J. Magn.

Magn. Mater. 155 (1996) 190.

[7] P.L. Cavallotti, N. Lecis, H. Fauser, A. Zielonka, J.P. Celis, G.

Wouters, J. Machado da Silva, J.M. Brochado Oliveira, M.A. S�a,

Surf. Coat. Tech. 105 (1998) 232.

[8] G. Zangari, P. Bucher, N. Lecis, P.L. Cavallotti, L. Callegaro, E.

Puppin, J. Magn. Magn. Mater. 157–158 (1996) 256.

[9] C. Yang, H.Y. Cheh, J. Electrochem. Soc. 142 (1995) 3040.

[10] C.A. Ross, L.M. Goldman, F. Spaepen, J. Electrochem. Soc. 140

(1993) 91.